Fuel Pump and Air Distributor arrangement for Reversing.

The fuel pump cam on the MAN B&W MC series engine is designed to raise the plunger on the injection stroke and then keep the plunger at the top of its stroke while the follower stays on the peak of the cam until just before the next delivery stroke when the follower returns to the base circle of the cam, and the fuel pump plunger moves down on its suction stroke.
The diagram on the top shows the cam follower just beginning to move up the slope of the cam with the camshaft rotating in anticlockwise direction. (i.e. start of injection)
If the engine direction is reversed at this point, then air will enter the pneumatic cylinder as shown and will move the piston to the right. The cam follower will be moved across and would finish in the position shown which would be at the correct fuel pump timing for running astern.

It should be noted that the reversal of the follower only takes place while the engine is rotating. If the engine had been stopped from running ahead, and then started astern, the fuel pump followers would move across as the engine starts to rotate, and before the fuel is admitted by venting the fuel pump puncture valves.

A micro switch shown on the left hand side detects whether the follower has moved across. If not, an indicator light is lit in the control room, however the engine will still start if a follower fails to move, perhaps due to corrosion in the servo cylinder a high exhaust temperature deviation alarm would operate within a short time. Allowing the engine to start in this situation could be useful during maneuvering in confined waters

AIR DISTRIBUTOR WORKING

The larger MC engines use a radial air start distributor operated by two cams fixed on the end of the camshaft. The pilot valves, arranged radially in a housing, are either pushed by the pilot air onto the inner cam profile when starting ahead, or onto the outer profile when starting astern. When not in use, a spring holds the pilot valve follower clear of both profiles, thus reducing wear.

Photo down below shows distributor mounted on end of camshaft. This is for a 12 cylinder engine, there is another distributor at the other end of the camshaft for the other 6 cylinders. No cover was removed to take this picture; There is actually a perspex window, so you can see the distributor!! 

There is one pilot valve for each air start valve. Each pilot valve has 3 inlet ports and one outlet port. When the engine is required to start, Pilot air at 30 bar from the main air start manifold is led to the middle inlet port.The air cannot pass through the pilot valve at this point because the port is shut off by the operating piston on the valve slide.

If the engine is required to start ahead then control air at 30 bar from the air start manifold is led to the top port. This acts on the top of the operating piston of the valve slide and forces the valve slide down onto the inner cam profile. As the slide moves downwards to admit pilot air to the air start valve, the top port is closed off and the main pilot air now acts on top of the actuating piston. As it does this, the pilot air passes through the exit port to actuate the air start valve in the cylinder.

As the cam profile moves the valve slide upwards, the piston on the operating slide closes off the pilot air and air to the air start valve is vented through the center of the valve slide.For astern operation the action is similar, but this time the control air enters through the lower port.